Low
S.21 on Heatline CaprizPlus 28

Your boiler is currently checking that its internal air extraction system is working correctly before it attempts to ignite the flame.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

What the manual says

Manufacturer manual

Pre-run fan

From this model's manual
Verified against the Heatline Capriz Plus installation & service manual, page 26.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

No action required; this is a normal operational status messageMonitor the display to ensure it progresses to ignition (S.4)If the code stays on screen indefinitely, try a resetIf the boiler fails to fire after this stage, contact a Gas Safe engineer

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